Hepatic sialic acid synthesis modulates glucose homeostasis in both liver and skeletal muscle

OBJECTIVE

Sialic acid is a terminal monosaccharide of glycans in glycoproteins and glycolipids, and its derivation from glucose is regulated by the rate-limiting enzyme UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE). Although the glycans on key endogenous hepatic proteins governing glucose metabolism are sialylated, how sialic acid synthesis and sialylation in the liver influence glucose homeostasis is unknown. Studies were designed to fill this knowledge gap.

METHODS

To decrease the production of sialic acid and sialylation in hepatocytes, a hepatocyte-specific GNE knockdown mouse model was generated, and systemic glucose metabolism, hepatic insulin signaling and glucagon signaling were evaluated in vivo or in primary hepatocytes. Peripheral insulin sensitivity was also assessed. Furthermore, the mechanisms by which sialylation in the liver influences hepatic insulin signaling and glucagon signaling and peripheral insulin sensitivity were identified.

RESULTS

Liver GNE deletion in mice caused an impairment of insulin suppression of hepatic glucose production. This was due to a decrease in the sialylation of hepatic insulin receptors (IR) and a decline in IR abundance due to exaggerated degradation through the Eph receptor B4. Hepatic GNE deficiency also caused a blunting of hepatic glucagon receptor (GCGR) function which was related to a decline in its sialylation and affinity for glucagon. An accompanying upregulation of hepatic FGF21 production caused an enhancement of skeletal muscle glucose disposal that led to an overall increase in glucose tolerance and insulin sensitivity.

CONCLUSION

These collective observations reveal that hepatic sialic acid synthesis and sialylation modulate glucose homeostasis in both the liver and skeletal muscle. By interrogating how hepatic sialic acid synthesis influences glucose control mechanisms in the liver, a new metabolic cycle has been identified in which a key constituent of glycans generated from glucose modulates the systemic control of its precursor.

A dominant negative ADIPOQ mutation in a diabetic family with renal disease, hypoadiponectinemia, and hyperceramidemia

Adiponectin, encoded by ADIPOQ, is an insulin-sensitizing, anti-inflammatory, and renoprotective adipokine that activates receptors with intrinsic ceramidase activity. We identified a family harboring a 10-nucleotide deletion mutation in ADIPOQ that cosegregates with diabetes and end-stage renal disease. This mutation introduces a frameshift in exon 3, resulting in a premature termination codon that disrupts translation of adiponectin's globular domain. Subjects with the mutation had dramatically reduced circulating adiponectin and increased long-chain ceramides levels. Functional studies suggest that the mutated protein acts as a dominant negative through its interaction with non-mutated adiponectin, decreasing circulating adiponectin levels, and correlating with metabolic disease.